[sdiy] Protecting microcontroller inputs from - voltage?

[The Old Crow]

On Sat, 8 Nov 2003, Bret Truchan wrote:

> Question #1: I'm assuming that it is _bad_ to allow negative voltage on the 
> inputs of a microcontroller. Is this a fair assuption?

  In general, yes.

> Question #2: What's the easiest way to clip out negative voltages?

  Use a diode clamp.  That is, a diode from the I/O pin to ground, with 
the cathode on the I/O pin.  Check the microcontroller electrical spec, 
but most will tolerate -1V or so.  If even less drop is needed, use a 
Schottky diode.  In practice, a 1N4148 will work fine.  Caveat: put a 
small resistor in series with the I/O pin and whatever it is sensing.
 
> > > Question #3: "stupid question": If I have some of the lines of the 
> microcontroller defined as outputs, I don't need to protect them from input 
> voltage, do I? If someone hooks up voltage to an output line, nothing will 
> happen, right?!?

  Use a series resistor (and clamp diodes if you wish).

Crow
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